How do I expand these 2 formulas: $$\Sigma_{s\in \{a,b\}}\Pi_{t\in \{c,d\}}x_{st}$$ and $$\Pi_{t\in \{c,d\}}\Sigma_{s\in \{a,b\}}x_{st}$$ ?
EDIT: They should be the same ! See $(6.29)$ and $(6.30)$ in the snippet below, but the are NOT.
$$x_{ac}x_{ad}+x_{bc}x_{bd}\quad (6.29)$$
$$(x_{ac}+x_{bc})\cdot(x_{ad}+x_{bd})\quad (6.30)$$
